Re: [PATCH] phiopt: Drop SSA_NAME_RANGE_INFO in maybe equal case [PR108166]

[Aldy Hernandez <aldyh@redhat.com>]

On Thu, Dec 22, 2022 at 11:30 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> The following place in value_replacement is after proving that
> x == cst1 ? cst2 : x
> phi result is only used in a comparison with constant which doesn't
> care if it compares cst1 or cst2 and replaces it with x.
> The testcase is miscompiled because we have after the replacement
> incorrect range info for the phi result, we would need to
> effectively union the phi result range with cst1 (oarg in the code)
> because previously that constant might be missing in the range, but
> newly it can appear (we've just verified that the single use stmt
> of the phi result doesn't care about that value in particular).
>
> The following patch just resets the info, bootstrapped/regtested
> on x86_64-linux and i686-linux, ok for trunk?
>
> Aldy/Andrew, how would one instead union the SSA_NAME_RANGE_INFO
> with some INTEGER_CST and store it back into SSA_NAME_RANGE_INFO
> (including adjusting non-zero bits and the like)?

if (get_global_range_query ()->range_of_expr (r, <SSA>, <STMT>)) {
  int_range<2> tmp (<INTEGER_CST>, <INTEGER_CST>);
  r.union_ (tmp);
  set_range_info (<SSA>, r);
}

Note that the <STMT> is unused, as the global range doesn't have
context.  But it is good form to pass it since we could decide at a
later time to replace get_global_range_query() with a context-aware
get_range_query(), or a ranger instance.  But <STMT> is not strictly
necessary.

Hmmm, set_range_info() is an intersect operation, because we always
update what's already there, never replace it.  If want to replace the
global range, throwing away whatever range we had there, you may want
to call this first:

/* Reset all flow sensitive data on NAME such as range-info, nonzero
   bits and alignment.  */

void
reset_flow_sensitive_info (tree name)
{
}

Aldy

>
> 2022-12-22  Jakub Jelinek  <jakub@redhat.com>
>
>         PR tree-optimization/108166
>         * tree-ssa-phiopt.cc (value_replacement): For the maybe_equal_p
>         case turned into equal_p reset SSA_NAME_RANGE_INFO of phi result.
>
>         * g++.dg/torture/pr108166.C: New test.
>
> --- gcc/tree-ssa-phiopt.cc.jj   2022-10-28 11:00:53.970243821 +0200
> +++ gcc/tree-ssa-phiopt.cc      2022-12-21 14:27:58.118326548 +0100
> @@ -1491,6 +1491,12 @@ value_replacement (basic_block cond_bb,
>                   default:
>                     break;
>                   }
> +             if (equal_p)
> +               /* After the optimization PHI result can have value
> +                  which it couldn't have previously.
> +                  We could instead of resetting it union the range
> +                  info with oarg.  */
> +               reset_flow_sensitive_info (gimple_phi_result (phi));
>               if (equal_p && MAY_HAVE_DEBUG_BIND_STMTS)
>                 {
>                   imm_use_iterator imm_iter;
> --- gcc/testsuite/g++.dg/torture/pr108166.C.jj  2022-12-21 14:31:02.638661322 +0100
> +++ gcc/testsuite/g++.dg/torture/pr108166.C     2022-12-21 14:30:45.441909725 +0100
> @@ -0,0 +1,26 @@
> +// PR tree-optimization/108166
> +// { dg-do run }
> +
> +bool a, b;
> +int d, c;
> +
> +const int &
> +foo (const int &f, const int &g)
> +{
> +  return !f ? f : g;
> +}
> +
> +__attribute__((noipa)) void
> +bar (int)
> +{
> +}
> +
> +int
> +main ()
> +{
> +  c = foo (b, 0) > ((b ? d : b) ?: 8);
> +  a = b ? d : b;
> +  bar (a);
> +  if (a != 0)
> +    __builtin_abort ();
> +}
>
>         Jakub
>